A while ago, I was wondering whether any reasonably intelligent person could behave as stupidly as the crook in my WIP, when the local paper ran this report: an accountant at the Oregon Dept. of Education embezzled nearly $1m without anyone suspecting a thing, which takes considerable cleverness; then another employee saw him stealing an office chair and reported it, leading to an investigation that brought the whole thing crashing down.
I read a lot of news stories that I wouldn't dare put into a book simply because readers would say the incidents were too far out to be believable. For instance, there was the bank robber in Springfield OR who efficiently robbed the chosen bank and ran away, only to trip when his baggy pants fell down around his ankles. He stepped out of them and continued to flee. A police dog found him hiding in the bushes in his underwear.
There were the burglars, in Glasgow IIRC, who parked their getaway vehicle in a dead-end alley. When they came out, alarms blaring, they tried to do a three-point turn and got stuck broadside across the alley.
Stories about bank robbers who write their hold-up notes on their own deposit slips--or something similar--and leave them behind when they run for it are anything but rare. A variation occurred locally when the paper used was the back of an application for a job at the bank that the man had already filled out, name, address, phone number and all.
A few years ago, a young man robbed the bank just around the corner from my house. He made his getaway by climbing over the chainlink fence between the bank and the school next door. When the cops arrived, the kids in the schoolyard all pointed and yelled, "he went that way!"--down a dead-end street, where he was found at his mother's house.
Anyone else read about the crooks in Australia who held up a restaurant? One of them accidentally fired his gun and hit the other in the buttocks. Then they grabbed a big sack and ran. What was in the sack? Bread rolls.
Murder isn't exempt from stupid crooks: What about the guy who shot his wife (age 51) and claimed it was a mercy killing because she didn't want the long suffering of her Lou Gehrig's disease? He had planned--he said--to kill himself too but didn't because her spirit returned to him and told him not to. Autopsy showed she didn't have Lou Gehrig's, in fact she was very healthy except for carpal tunnel. And the doctor he claimed had diagnosed the disease denied it absolutely.
In a mystery, your villain is supposed to be reasonably intelligent, reasonably rational, reasonably capable of hoodwinking the sleuth. Real life ain't like that!